1. Field of the Invention
The present invention relates in general to a resistor circuit for use in semiconductor integrated circuits, and more particularly to a layout arrangement for fabricating a voltage division circuit portion.
2. Description of the Related Art
A voltage division circuit portion for dividing a voltage developed across a terminal A and a terminal C for use in semiconductor integrated circuits, as shown in FIG. 2, includes a reference resistor portion Rref, a resistor portion consisting of several resistor elements with which a desired resistance value can be adjusted, and trimable fuses having the same number as that of the resistor elements for being used in order to determine a desired resistance value. Here, when the voltage developed across the terminals A and C is V, the voltage developed across the terminals A and B is VAB, a resistance value of the reference resistor portion between the terminals A and B is Rref, the voltage developed across the terminals B and C is VBC, and a resistance value of the resistor portion between the terminals B and C is RBC, the voltages VAB and VBC which are obtained by dividing the voltage developed across the terminals A and C are expressed by the following Expressions, respectively:
VAB=Vxc3x97Rref/(Rref+RBC)xe2x80x83xe2x80x83(1) 
VBC=Vxc3x97RBC/(Rref+RBC)xe2x80x83xe2x80x83(2) 
Since the voltage V developed across the terminals A and C is divided into VAB and VBC, the voltage developed across the terminals A and B and the voltage developed across the terminals B and C can be obtained on the basis of the resistance ratio of Rref to RBC. The value of the resistance between the terminals B and C for which the desired resistance value is required is as follows. In general, in the trimming of a resistor, the value of the resistance between the terminals B and C is adjusted to the desired resistance value. In this case, the trimming elements F1 to F4 are connected in series with one another, and also the resistors R1 to R4 are connected in parallel with the respective trimming elements. Here, it is assumed that the resistance values of the trimming elements F1, F2, F3 and F4 are RF1, RF2, RF3 and RF4, respectively. If the parallel resistance value of the trimming element F1 and the resistor R1 is assumed to be RF1//R1, then the value of the resistance between the terminals B and C is expressed by (RF1//R1)+(RF2//R2)+(RF3//R3)+(RF4//R4). In addition, in general, the resistance values RF1 to RF4 of the trimming elements F1 to F4 are negligibly smaller than the resistance values of the resistors R1 to R4. Consequently, the value of the resistance between the terminals B and C becomes (RF1+RF2+RF3+RF4) in the state in which each of the trimming elements is not cut. Here, if the trimming element F1 is cut, then the value of the resistance between the terminals B and C becomes (R1+RF2+RF3+RF4). However, since the resistance values of RF2, RF3 and RF4 are each negligibly smaller than the resistance value R1, the value of the resistance between the terminals B and C becomes roughly R1.
In addition, if the trimming elements F1 and F2 are both cut, then the value of the resistance between the terminals B and C becomes (R1+R2+RF3+RF4). However, since the resistance value (RF3+RF4) is negligibly smaller than the resistance value (R1+R2), the value of the resistance between the terminals B and C becomes roughly (R1+R2). Consequently, arbitrary one or more of the trimming elements F1 to F4 are cut, which makes it possible to adjust the value of the resistance between the terminals B and C to a desired resistance value. However, in the prior art, no arrangement of a layout pattern in the process of fabricating the above-mentioned voltage division circuit is taken into consideration at all.
The resistor portion of a semiconductor integrated circuit is made in the form of a transistor or high-resistance polysilicon, through the diffusion, or so forth. In this case, the resistor portion will hereinbelow be described by giving one made of high-resistance polysilicon as an example. The high-resistance polysilicon is made by the ion implantation and the resistance value thereof can be set on the basis of an area of a resistor element made of high-resistance polysilicon. However, since there is nonuniformity in the ion implantation and this nonuniformity is actualized in the form of the in-plane fluctuation of a chip, it is difficult to realize a resistor portion having a resistance value which is highly accurate and uniform in the whole surface of a chip.
The voltage division ratio in the voltage division circuit is determined by the ratio of the resistance of the reference resistor portion to the resistance of the resistor portion having a desired resistance value, which is the factor of generation of the large error in the resistance value in the case where the reference resistor portion and the resistor portion having a desired value are arranged separately from each other within the chip without taking the layout into consideration as in the prior art. Since the influence of the in-plane fluctuation in the resistance values due to the nonuniformity in the ion implantation is actualized in the form of the resistance-ratio error, it is difficult to realize the highly accurate voltage division circuit.
In the light of the foregoing, the present invention has been made in order to solve the above-mentioned problems associated with the prior art, and it is, therefore, an object of the present invention to provide a resistor circuit with which a resistance ratio among voltage division resistors in a semiconductor integrated circuit can be realized with high accuracy.
In order to attain the above-mentioned object, the present invention may provide that in the arrangement of a layout pattern of a voltage division circuit for use in semiconductor integrated circuits, resistor elements are arranged in order from a resistor element having a largest resistance value in the periphery of a reference resistor portion.
Therefore, according to the present invention, there is provided a resistor circuit, comprising:
a reference resistor portion; and
a resistor portion including resistor elements and fuse connected in parallel with the resistor elements, respectively, for trimming the resistor elements,
in which the resistor elements are arranged in order from the resistor element having the largest resistance value so as to be adjacent to the reference resistor portion in the periphery of the reference resistor portion.